Means for overcoming the effects of earth reflections in directional antenna systems



2 Sheets-Sheet 1 April 1947- E. H. ULLRIGH MEANS FOR OVERCOMING THEEFFECTS OF EARTH REFLECTIONS IN DIRECTIONAL ANTENNA SYSTEMS Filed Sept9, 1942 INVENM/P Maxim" ATTORNEY 2,419,609 CTIONS IN A ril '29, 1947.

MEANS FOR OVERCOMIN E. H. ULLRICH G THE EFFECTS OF EARTH REFLE whateverits angle' of elevation.

Patented Apr. 29, 1947 UNI-TED smres PATENT omen- EARTH REFLECTIONS INDIRECTIONAL ANTENNA SYSTEMS Edward Hill Ullrich, London" W. e. elegant;assignor to Standard Telephones and Cables Liniited, London, England; aBritish j'cojmp'anfr ApplicationS'eptember 9, 1942', Serial N 4571861 InGreat Britain September 25, 1940 6 claims.

This-invention relates toj dir'jectiona'l maidentenna systems of thekind used for defining a radio course by theoverlappingialong'saidcourse" 2 shoWs a method for determining the effectsfof reendreneenonupon a" single directive' radiation pattern N, by naming the" imagereflection N; N

Fig. 3 shows thedistor'tionoi the radiation pattern due to ground effectof a pair of anteiifriasys't'emsspaced vertically apart 60;

Fig. 4 shows a side elevation of an embodiment of abplicants invention;and

id is a perspective View of the lower end of'the antenna array shown inFig. 4, theview being taken from the left of Fig. 4 and at a slightelevation.

It is sometimes necessary to establish in the skif'a straight radiocbursealong the axis; oi a mechanical antenastructure, and to ensurethat th" curse remains accurately along thisaxis Brecision requirementsusually dictate the use overlapping antenna directivitydiagrams as Y andZ pflfig; lot the drawings accompanying the present-specification, thecoursevia OP'in Fig: 1 being that along which equal sign als;are received from two different aerial arrays (which may be separate structures or which may utilise the same antenna element's differentlyexcited). If such a system as that shown inFigure- 1 were set up andmade to operate satisfactorily at a causingthe course to deviate fromits neespaee directidnl Theantenna characteristic shown in Fig. 2: ofthe drawings ac'cbmpanymgahe prfesentspecification where'for clearnessonly one antenna diagram of 'the two overlapping diagranis'is shown;

is' 'that relating "to free space; the"'m'ain beam" being directed alongthe axis'OQ. Any'ray 0B1 L -v' 1 which aiter refl CtiOnT tr vels along,a path" 30 parallel to 0A interferes at. r elatively great di's f tanceswith'the ray 0A; The'shape, of the lobes is thus modified, sothat'theline of intersection: 5 in Fig. 1 is'nolonger OP, or,'injothi" .w"ofds,'niej course hasshifted. It is required top'revent this change of course.i A p The effect of] the'earth is to produce" reflected rays which maybe. considered, as coining mm; the imageof the original aerial systein,and the signal at a distance will be the resultant of'the' signalsproduced byQthe two systems; flfhe'ide'al. solution would e; to reducean. econdary: ib esjf of'the'directivity curve to negligible size andthus; avoid disturbing reflections altogether, but this involvesabnormally great aerial dimensions at wavelengths of the order of 1metre. 7

It is" important to note here thatif the phase of the various elementsof an antenna arrayi s" reckoned relativelyto a point of s'y'rhmetryofthe antennastruc'ture.and such a point of sy'rniiie try exists'inall"the more commonly used aerial systems--a11 rays of the directive"diagram are in phase or exactly out of phase. Furthermore; the phase'ofany ray of one lobeis, general, opposite to that of a ray of anyadjacent lobe; Thus, if in Fig; 2 of the drawings acco pany n thepresent specification the phase of rays in lobe N is.'cal'ledpositive,the phase of rays inlpbe L will be negativeTand thatof raysin lob'e'Magain positive. It is" only in spec'ial structuresf representing-{forexample, a" limiting; cas'eiin which a lobe suchas'L-has been reducedto'ze ib' dimensions'that two adjacent lobes have the same sign. a I

In Fig. 2 the free-space direotivity"diagranifis" shown with lobes,such'asN, LandM and'the reflection" image directivity diagram with lobesSuCh'ets N, L and M. A O'B, refi titd 'fi'bfn 40 the gr'oundatB; appearstoarrive'fr'oiii 0' f0 lowing the straight line OBC. T ere is*'a'pha'sechange atrefiection which depends on 'the na ture of the-terrain; in thecase ofaiifidflc'ofi ducting reflector it is -180 degrees:

Aphase change of 180 will be a's'suiiied} The" image aerial excitationmust; then; 'befconsid ered as-loeing degrees out of phase with-theft ofthe real aerial. Therefle'cted ray -ifiany dij rectionBC (see Fig"; 2),have, with respect to'th'e' radius vector OP of diagram Z must beaffected equally by the reflected rays. In general, however, thereflected ray parallel to OP has difierent amplitudes for the images ofthe Y and Z diagrams respectively. It may even have opposite igns.Furthermore, these relative ampli tudes and signs vary with theelevation.

It is possible to mitigate this trouble by changing automatically thespacing or the excitation of the antenna elements as the elevation ischanged, doing this in uch away as to afiect the main ray OP equally inboth positions Y and Z of the directivity diagram. This is, however, acomplicated process. I f

A impler method according to thisinvention is to choose an aerial, thesecondary lobes of the free pace diagram of which are all small com-.pared with the main lobe, and to adjust the height above earth of thecentre of this aerial in such a way that the phase change correspondingto O'D of Fig. 2 is an odd multiple of 90. degrees. The direct andreflected ray vectors will then be at right angles, and if the reflectedray is ai'small quantity of the first order relative to the, direct ray,the. resultant amplitude will differ from the amplitude 01' the directray by a small quantity'of the second order.

Let d be the height of the centre of the anten'na above earth (see Fig.2) and the angle made by the course with the vertical. Then OTD=2d cosIt is, then, clear that, in order to ensure that the, main and reflectedrays add at right angles, the :height d, above earth must vary with sothat Where n'is any integral number operating wavelength.

It will be noted that the height 01 need not be more than doubled for achange of course from zero to sixty degreeswith the vertical. If smallerheight variations are required, the value of n may be changed abruptlyanywhere.

I Irr order tofix the position of earth, a conducting; mat-may be. putunder the mechanical structure. The largerthe mat, the moresatisfactory-the operation; To cover an. elevation variation fromvertical to sixty degrees from the vertical, a copper mat might be triedextending on each side of the base of the structure by a distance atleast four times the height of the highest element, and preferably more,

Assume for example that it is required to sweep the course over an anglevarying from zero to 45 degrees with the vertical.

Two antenna arrays for example of the Yagi type may be used eachconsisting of a row of eleven dipoles spaced M4 and excited by astraight transmission line. Each dipole of any one array is thus excitedwith current having a phase lag of 1r/2 with respect to the dipoleimmediately preceding it. The directivity diagram is that given in Fig.3 of the drawings acijsmpanying the present specification showing a mainlobe F and successive smaller lobes F, F, F' and F"". If the maximumamplitude of the main lobe is eleven, that of the other successive and Ais the lobes over 180 degrees are, is 2.45, 1.53, 1.2, 1.04

and 1.

The two antenna arrays resulting in a radiation pattern as illustratedin Fig. 3 are set at 60 degrees to one another and in the same. plane,and their height is determined'in accordance with the law given above.They are, excited al- 4 ternately and must not have appreciablecoupling.

The equi-signal course, indicated at E in Fig. 3, is then the bisectorof the angle between the two arrays which produce diagrams E and G, andthe amplitude along the course is .6 if that of the main lobe is eleven.When the course is at an angle 0 with the vertical, the reflected ray isat an angle 1r-26 with the course. The maximum change of amplitude ofthe course ray is w since in the case' of one of the diagrams thereflection come from the axis of the main secondary lobe of amplitude2.45. In Fig. 3 the chain-dotted curve H is that part which is ofinterest of the diagram after distortion by the reflected ray; thedotted curve G is the diagram of the conjugate Yagi array; it suffersnegligible distortion due to reflections. It can be seefi'from Fig. 3that reflection has thus shifted'thej course about half a degree only.When 0 is l'ess than 45, the shift is always less, usually considerablyless. If it is desired to work with a greater than 45?,

it is necessary to' use 'a more directive diagram than that of Fig. 3 i.e., to have more than eleven dipoles in this type of array. Theprinciples above described apply equally to a course set up in space bytransmitting aerials and to a homing course corresponding to receivingaerials. The directivity curve of a horizontal dipole distance bx abovea perfect conducting earth is.

given by:

=sin 21rb sin 0) where P is the radius vector at 0 with the vertical.

the directivity diagramof an aerial system is' being calculated that, ifthe directivity curve of an antenna made up of a number of similarelements is given'by:

posite element (say a pair of dipoles) having a directivity curve r=F(a)then the directivity curve of the new system will 7 be given by:

r=j(a) -F(a) one having zero radiation in that direction.

One such composite element is a pair of parallel dipoles separated aquarter of a wavelength,

If, then, it is arranged that, as the elevation of the array changes,the direction AB is also changed E I so as to coincide with thedirection of departure of the ray, which after reflection at the earthwill be parallel to the radio course, it is ensured that this ray isalways of zero intensity and the equi-signal course of the overlappingdiagrams unchanged,

The system shown in Fig. 4 of the drawings accompanying the presentspecification consists of four row of horizontal dipoles:

These four rows of horizontal dipoles are parallel to each other and arearranged at an angle of elevation AE which is adjustable by moving thearray to different angular positions in the vertical plane of the array.The axis of each dipole is normal to the plane of the paper in Figure 4.The lower end of the array composed of the four rows of horizontaldipoles is shown in perspective in Figure 4a. The Bs and Us arereflectors spaced a quarter of a wavelengthfrom the corresponding As andUs respectively and so oriented that the vertical bisects the anglebetween the radio course on the one hand and the directions BA and DC onthe other. Each dipole pair, such as dipoles A11 and B11, constitutes anelementary antenna unit having a direction of zero radiation indicatedby the arrow marked Zero radiation. The direction of the courseestablished by the array is indicated by the arrow Course in Figure 4.As explained above, the dipole pairs are oriented so that the anglebetween the Course direction and the direction of zero radiation isbisected by the vertical line in Figure 4 marked Vertical. In otherwords, the two angle marked 0, are equal. The refiectors may be excitedby radiation only. The A and C series are each excited from a source HFthrough lines Ta and To as in the arrangement described with referenceto Fig, 3, but the C series is excited one firth of a cycle out of phasewith series A, the phase being made to alternate between lagging andleading so as to obtain overlapping directivity curves, as by means ofsuitable phase shifting devices marked 72+ and 72 included in alternateenergizing circuits for the C row, of dipoles. The spacing of A row fromthe C row, and therefore, of the B row from the D row is four fifths ofa Wavelength. The orientation of any elements An B11 and Cu Dn iseffected by pivoting them about the centre points Pn, Q11 respectivelyof An B11 and Ch Dn.

The dotted lines joining the centers of the dipoles of each pair andincluding the pivot points P1, P2 Pu, and Q1, Q2 Qn, represent suitablepivotal mountings for the dipole pairs by which the pairs may beadjusted to different angular positions within the vertical plane inwhich the radio course is established.

The above solution thus consists in forming an aerial array from aplurality of elementary antenna systems, each of which has a directionof zero radiation and providing means for rotating, when the directionof the course is changed, each of the elementary systems in such a waythat the vertical is the bisector of the angle between the coursedirection and the direction of zero radiation of the elementary systems.

What is claimed is:

An arrangement for defining a radio course in a vertical plane and at apredetermined angle of elevation comprising an antenna system forproducing two overlapping'fadiation diagrams the intersection of whichforms said radio course,

said antenna system comprising a plurality of elementary antenna unitseach of which has a direction of zero radiation, the effects of earthreflection being overcome by orientation of said elementary units sothat a vertical line bisects the angle between the defined coursedirection and the direction of zero radiation of said elementary units,

2. An arrangement for defining a radio course in a vertical plane and ata predetermined angle of elevation comprising an antenna array includingtwo parallel rows of dipole pairs arranged in the same vertical planeeach dipole pair comprising a driven dipole and a parallel reflectordipole and the dipole pairs of each row being spaced one quarter of awavelength apart, means for feeding one of the two rows of drivendipoles with current having alternately a leading and lagging phase withrespect to current fed to the other row so as to produce two overlappingradiation diagrams for defining the course, and means for mounting eachdipole pair for angular adjustment in said vertical plane to a pointsuch that a vertical line bisects the angle between the defined coursedirection and the direction of zero radiation of said dipole pairs.

3. An arrangement according to claim 2 wherein the driven dipole andreflector dipole of each dipole pair are spaced one quarter of awavelength apart.

4. An arrangement according to claim 2 wherein said rows of drivendipoles are spaced four-fifths of a wavelength apart.

5. An antenna array for defining a radio course in a vertical plane andat different. adjustable angles of elevation comprising a plurality ofelementary antenna system each having a direction of zero radiation,said array being mounted for movement to different angles of elevationin said vertical plane, and means for mounting each elementary antennasystem for angular adjustment in said vertical plane to a position suchthat, for each angular position of said array, a vertical line bisectsthe angle between the defined course and the direction of zero radiationof the elementary antenna system.

6. In an antenna a ray for defining a radio course in a vertical planeand at different adjustable angles of elevation by means of a pluralityof elementary antenna units each of which has a direction of zeroradiation, the method of overcoming the effects of earth reflectionwhich consists in adjusting the angular position of each elementaryantenna unit for each angular direction of said course so that avertical line is the bisector of the angle between the defined courseand the direction of zero radiation of the antenna unit.

EDWARD HILL ULLRICH.

REFERENCE S CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,227,929 Goddard Jan. 7, 19412,127,954 Dunmore Aug, 23, 1938 2,294,882 Alford Sept. 8, 1942 1,885,151Sterba Nov. 1, 1932 2,038,539 Carter Apr. 28, 1936 2,186,554 PerrouxJan. 9, 1940 2,194,741 Davies Mar. 26, 1940 2,268,107 Bond Dec. 30, 1941

